The invention relates to a portable identification transmitter for a passive access system of a motor vehicle, which has in a housing a supply battery, a microcontroller, HF transmitting and receiving circuits, which are controlled by the microcontroller, for communication with a vehicle-sided control unit, and an LF receiving circuit, which is coupled to the microcontroller, for receiving LF wake signals transmitted in the LF frequency range and for transmitting a wake signal to the microcontroller.
In addition, the invention relates to a method for an energy-efficient operation of such a portable identification transmitter.
Such portable identification transmitters, which are also called electronic keys or ID transponders, have the purpose of allowing an owner or user of the identification transmitter access to or the possibility of locking a motor vehicle, and the user does not actively have to press enter keys on the identification transmitter. The user only has to carry the identification transmitter (for example, in his clothes or a handbag).
These functionalities are called “passive keyless entry systems” or “keyless go systems”. In contrast to conventional remote control, the “passive keyless entry functionality” requires no active actuation of the mobile identification transmitter for locking and unlocking (securing and releasing) the locking device or for unlocking the immobilizer system. For example, when actuating the door handle of the motor vehicle, communication is started between the motor vehicle and the identification transmitter and with positive identification verification (authentication) the power release of the locking device is activated. As a result, the user that carries a valid identification transmitter can open his motor vehicle without actively actuating the identification transmitter.
For example, a desired process of accessing a motor vehicle can take place as follows: the user carrying the identification transmitter approaches the driver door of the motor vehicle and reaches behind the door handle. At this moment the approach of the user or his action of reaching behind the door handle is detected by a sensor which is attached at the door (for example, by a capacitive proximity sensor mounted in the door handle) and signaled to a vehicle-sided control unit. Naturally even when a user is not authorized, i.e., when he does not carry a suitable identification transmitter, such a user would be detected and a corresponding signal would be given. An approach detection and corresponding signal would also be given when any other body approaches the sensor in a particular manner. Therefore, an access authorization has to be provided before the door is unlocked by the vehicle-sided control unit and the door handle is released for opening the door. For this purpose, an exchange of radio signals takes place which results in a dialog between the vehicle-sided control unit and a microcontroller installed in the identification transmitter. Basically, this dialog could start with an HF request signal on the part of the vehicle-sided control unit, followed up with an HF response signal emitted by the identification transmitter. However, in this case, the identification transmitter would have to contain a constantly active HF receiver. Another disadvantage of such HF communication would involve the difficulty of developing a clearly defined spatial reception range in the proximity of the motor vehicle. To ensure that the identification transmitter is activated only when it is in the predetermined spatial range (adjacent to the motor vehicle door), it is commonly practiced in prior art that the vehicle-sided control unit first of all emits via an LF transmitter with an associated transmitter coil a wake signal in the LF frequency range (for example, 125 kHz), wherein the transmitter coil is arranged (for example, in the door handle) in such a way that a predetermined dispersion range of, for example, a few meters, is created. On the other hand, the identification transmitter is provided with an associated LF receiver which has receiver coils in addition to the LF amplifier circuits.
An output of the LF receiver is coupled to an input of the microcontroller. When now the identification transmitter carried by the user and provided with an LF receiving circuit is present in the dispersion range of the LF transmitter coil of the vehicle-sided control unit when the user reaches for the door handle, the identification transmitter receives a wake signal in an LF frequency range which is immediately emitted by the vehicle-sided control unit via the transmitter coil as soon as the action of the user or his approach is detected. The LF wake signal received has the purpose of signaling the microcontroller and initiating in program-controlled manner the HF signal dialog with the vehicle-sided control unit. Therefore, the microcontroller and the HF transmitting and receiving circuits can remain changed down to low energy consumption in idle mode. However, the LF receiving circuit must remain activated (active) to be available for receiving the wake signal.
However, this constant stand-by of the LF receiving circuit results in energy consumption and thus causes the battery to discharge. Therefore, the patent application DE 10 2010 036 787.7 made the proposal to switch the LF receiving circuits off and briefly activate them only in preset intervals.
Furthermore, from EP 0808971 it is known to provide the portable transceiver with a motion switch.
However, despite those proposed measures, in conventional ID transponders a voltage source keeps discharging when not in use. Especially when after being manufactured an ID transponder is stored and not used for longer periods of time before it is delivered and coupled to a motor vehicle, it is possible that the battery is discharged to a considerable degree by the time the ID transponder is delivered. Furthermore, when activated after being manufactured such ID transponders can cause undesired interactions with the surrounding area, for example during transport or in storage.